(1) Field of the Invention
The present invention relates to apparatus for bonding terminals of semiconductor devices to metal leads of plastic film carriers, more specifically to a flexible support stage that helps assure that a uniform pressure is applied to all leads and terminals during the actual bonding operation.
(2) Description of Prior Art
Tape automated bonding (TAB), though proposed many years ago, has finally found a niche in applications requiring chip connections beyond the capability of wire bonding. TAB provides the density for handling the high input/output terminal counts of VLSI chips, thereby keeping chip, or die, size to a manageable level for attaining a reasonable wafer yield. The driving force for TAB development and eventual use is high density. Wire bonding has reached its limit at bonding wires to pads on 6 mil centers along the periphery of the die. For example, a 300 input/output die, i.e. 75 bonds per edge, requires a die size of about 0.450 inches squared. The same application using TAB, where 4 mil contact centers are common (2 mil metal: 2 mil space), requires a reduced die size of about 0.300 inches square. This smaller die size allows approximately 120 more die per 5 inch wafer, while also offering a smaller device for higher packaging density.
When the number of input/output terminals on a chip to be bonded to the plastic film carrier or tape is low, the relative size of the gold bumps on the chips is relatively large, and the process is simple and reliable. However, as the number of terminals increases, the device becomes larger, and the gold bumps become smaller, the tolerances for producing complete bonding of all leads decreased significantly. The lace of chip planarity and the inherent variations in the gold bumps height has been the most troublesome aspects of the gang bonding of high input/output devices, in general, when the number of input/outputs exceed 200. The results can be non-bonded leads, and cracked chips.
In FIG. 1 there is illustrated a Prior Art TAB bonding apparatus wherein a device input/output, provided with input/output terminal bumps, is supported on a base plate 14. A plastic film carrier 16, provided with leads 18, is located over device 10 with the ends of leads 18 positioned over bumps 12. A thermode 20 is shown positioned over the device 10 in bonding position. The lower surface of thermode 20 is in a plane that is parallel to the plane defined by the tops of bumps 12. This relationship is the desired one which will result in good bonds between the leads 18 and bumps 12, and also will not expose harmful stresses on the device 10, because the pressure applied to the device by the thermode 20, will be uniform.
In FIG. 2 there is illustrated the same bonding apparatus, but where the plane of the lower surface of the thermode 20 is not in a plane that is parallel to the plane defined by the tops of bumps 12. As indicated, when the bonding operation is carried out the leads 18 on the right side, not in contact with thermode 20, will not be bonded to bumps 12, and cracks 22 in the device can result, because the stress is extreme.
This problem has been recognized, and efforts to remedy it include providing an adjustable support plate, as illustrated in FIG. 3. Here, the base plate 14 is mounted on an adjustable support plate in turn mounted on frame 24. The plane of the top surface of support plate 22 can be adjusted so that the plane defined by the tops of bumps 12 is parallel to the plane of the lower surface of thermode 20. This is accomplished by providing dowels 26, that extend from plate 24 into slidable relation within apertures 28 of plate 23. Plate 23 is thus vertically movable relative to plate 24, with very limited tilting permitted. A plurality of adjustabile screws, each provided with a head 32, seated in an enlarged hole 34, are used to hold the plate 23 in a desired plane. This arrangement accommdates an adjustment for non planarity between the device and thermode, but requires a painstaking manual adjustment.
Other prior art covering the TAB process can be found in ANONEWS Vol. 9, No. 4, June 1990, Page 1, published by ANORAD Corp. of Hauppauge, N.Y. and "Tab Tames High Density Chip Input/Outputs", Electronic Packaging and Production, December 1988, Pages 42-44.